54,148 research outputs found
Propagation of sound waves through a linear shear layer: A closed form solution
Closed form solutions are presented for sound propagation from a line source in or near a shear layer. The analysis was exact for all frequencies and was developed assuming a linear velocity profile in the shear layer. This assumption allowed the solution to be expressed in terms of parabolic cyclinder functions. The solution is presented for a line monopole source first embedded in the uniform flow and then in the shear layer. Solutions are also discussed for certain types of dipole and quadrupole sources. Asymptotic expansions of the exact solutions for small and large values of Strouhal number gave expressions which correspond to solutions previously obtained for these limiting cases
Matter Wave Scattering from Ultracold Atoms in an Optical Lattice
We study matter wave scattering from an ultracold, many body atomic system
trapped in an optical lattice. We determine the angular cross section that a
matter wave probe sees and show that it is strongly affected by the many body
phase, superfluid or Mott insulator, of the target lattice. We determine these
cross sections analytically in the first Born approximation, and we examine the
variation at intermediate points in the phase transition by numerically
diagonalizing the Bose Hubbard Hamiltonian for a small lattice. We show that
matter wave scattering offers a convenient method for non-destructively probing
the quantum many body phase transition of atoms in an optical lattice.Comment: 4 pages, 2 figure
Topology of the polarization field in ferroelectric nanowires from first principles
The behaviour of the cross-sectional polarization field is explored for thin
nanowires of barium titanate from first-principles calculations. Topological
defects of different winding numbers have been obtained, beyond the known
textures in ferroelectric nanostructures. They result from the inward
accommodation of the polarization patterns imposed at the surface of the wire
by surface and edge effects. Close to a topological defect the polarization
field orients out of the basal plane in some cases, maintaining a close to
constant magnitude, whereas it virtually vanishes in other cases.Comment: 4 pages, 3 figure
Surface-slip equations for multicomponent nonequilibrium air flow
Equations are presented for the surface-slip (or jump) values of species concentration, pressure, velocity, and temperature in the low-Reynolds number, high-altitude flight regime of a space vehicle. The equations are obtained from closed form solutions of the mass, momentum, and energy flux equations using the Chapman-Enskog velocity distribution function. This function represents a solution of the Boltzmann equation in the Navier-Stokes approximation. The analysis, obtained for nonequilibrium multicomponent air flow, includes the finite-rate surface catalytic recombination and changes in the internal energy during reflection from the surface. Expressions for the various slip quantities were obtained in a form which can be employed in flowfield computations. A consistent set of equations is provided for multicomponent, binary, and single species mixtures. Expression is also provided for the finite-rate, species-concentration boundary condition for a multicomponent mixture in absence of slip
Prediction of the functional properties of ceramic materials from composition using artificial neural networks
We describe the development of artificial neural networks (ANN) for the
prediction of the properties of ceramic materials. The ceramics studied here
include polycrystalline, inorganic, non-metallic materials and are investigated
on the basis of their dielectric and ionic properties. Dielectric materials are
of interest in telecommunication applications where they are used in tuning and
filtering equipment. Ionic and mixed conductors are the subjects of a concerted
effort in the search for new materials that can be incorporated into efficient,
clean electrochemical devices of interest in energy production and greenhouse
gas reduction applications. Multi-layer perceptron ANNs are trained using the
back-propagation algorithm and utilise data obtained from the literature to
learn composition-property relationships between the inputs and outputs of the
system. The trained networks use compositional information to predict the
relative permittivity and oxygen diffusion properties of ceramic materials. The
results show that ANNs are able to produce accurate predictions of the
properties of these ceramic materials which can be used to develop materials
suitable for use in telecommunication and energy production applications
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